1. Field of the Invention
This invention relates to a system for transforming metal surface oxides to metal. More particularly, this invention relates to a system for preparing metal surfaces for dry soldering and solder fellow processes.
2. Description of the Prior Art
Dry soldering and solder fellow processes, in general, can be used as alternatives to solder processes where wet fluxes are used. These dry processes are compatible with vacuum and controlled ambient processing technologies used for soldering electronic components. Dry soldering and solder reflow processes are especially, but not exclusively applicable to the manufacture of printed circuit boards, semiconductor chip lead attachment and packaging multichip modules. For these applications, it is desirable to minimize potentially corrosive residue by-products of the solder reflow process. In general, in any manufacturing process, safety, and environmental impact are of practical concern. Dry soldering and solder reflow processes can be tailored to self consistently fulfil these various concerns.
Dry soldering and dry solder reflow processes typically involve a reaction between a gas or vapor and a metal oxide that reduces the metal oxide and produces volatile oxide by-products which are removed by gaseous flow.
Prior work in the area of dry solder reflow has centered on the use of chemically reactive, halogen-containing gases such as CF.sub.2 Cl.sub.2, CF.sub.4, and SF.sub.6. Such gases leave halide residues which can reduce solder bond strength and promote corrosion. In addition, gaseous halide compounds represent potential safety and environmental contamination problems. Halides can attack soldering system fixturing, the associated vacuum pumping system and organic compound substrates used for printed circuit boards.
Inductively coupled and capacitively coupled, direct current, audio frequency, radio frequency and microwave frequency gas plasmas can be used to produce reactive radicals with enhanced chemical reactivity for dry soldering and solder reflow processes. Such techniques are similar to commercial, plasma assisted reactive ion etching processes commonly used by the microelectronics community to fabricate integrated circuits. Such processes are flawed by the inherently diffuse nature of a plasma. The fundamental laws of the conduction of electricity through gases impose conditions on the size, shape and geometry of the electrodes and the walls of such systems and the temperature of the piece-part. It is difficult to localize and direct a plasma to the specific area where it is needed to solder small substrates. Many reactive gas plasmas attack fixturing in the vacuum chamber. If the plasma potential is not actively controlled, ions from the plasma will physically sputter material and/or give rise to space charge accumulation on electrically isolated surfaces. Physical sputtering and space charge accumulation can injure or destroy certain electronic components such as field effect transistors and capacitors.
A recent U.S. patent to Pedder et. al., U.S. Pat. No. 5,000,819, 1981, specifically describes a system employing atomic hydrogen produced in a microwave frequency plasma for the purposes of metal surface cleaning and dry solder reflow. Microwave systems with sufficient power to dissociate molecular hydrogen are relatively expensive, and represent potential safety problems associated with radiation leakage and eye damage arising from exposure to ultraviolet emissions from the plasma. Furthermore, microwave radiation can harm certain electronic components. Substrate temperature control can be a formidable problem in microwave plasma systems. In general, atomic hydrogen produced in a microwave plasma is characteristically distributed over a large area and not efficiently utilized. This lack of spatial control can lead to damage of peripheral areas on piece-parts that are not directly involved in the soldering process.
Other dry soldering and solder reflow approaches employ one or more lasers to heat the near-surface region of a metal with a layer of metal oxide, and thermally vaporize or ablate the oxide layer to prepare the material for soldering. Laser ablation is currently also of interest to the integrated circuit fabrication community as a lithographic technique. In general, it is believed that in addition to thermal vaporization, laser ablation processes also involve photo assisted desorption processes to some extent. Additional lasers may be used to heat the deoxidized metal to temperatures required to melt and flow the solder. A practical laser ablation dry solder reflow process typically requires a controlled ambient with an inert gas such as argon or a reducing gas such as nitrogen and/or hydrogen to inhibit reoxidation of the native metal surface by residual contaminants or by vaporized oxide compounds. Ambient control is necessary to expand the process window and frame workable limits. Otherwise, the characteristically low melting point and boiling point of a metal vs. its metal oxide make laser ablation an extremely delicate process. A laser ablation dry solder fellow process is constrained by a number of considerations. Lasers are relatively expensive and relatively inefficient. In order for this technique to work there must be a clear line of sight between the laser and the part which means the (1) the solder must be deoxidized and mechanically placed on the deoxidized metal surface in separate operations and (2) the window into the controlled ambient system and associated optics in the system must be kept clean of condensed reaction by-products.